The eventual commercial production of shale oil in sufficient quantities to constitute a significant replacement of petroleum oil will involve the handling of enormous quantities of inert inorganic mineral refuse in the process of recovering the kerogen content from the oil shale. For example, commercially recoverable oil shale generally contains from about 85 percent to about 95 percent mineral matter, with the kerogen-rich material constituting a very minor proportion of the overall in-place oil shale. This large amount of inorganic mineral matter interferes with subsequent processing in a number of ways. For example, in retorting the shale, very large and/or numerous retorts are required to handle the commercial quantities involved. Furthermore, a substantial quantity of heat is expended and lost in heating up the shale to retorting temperature and cooling it down. Additionally, the retorting procedure is a source of contaminating fines, the greater the quantity of shale, the greater the quantity of fines. A further source of pollution is the spent shale recovered from the retort. In the process of retorting, a multitude of chemical reactions are caused to occur in the shale in the process of volatilizing the kerogen. This results in a remnant of chemical compounds in the spent shale leaving the retort. Since these remnant compounds are not naturally occurring, they constitute a potential environmental pollutant in the discarded shale and present a particular hazard in surface water pollution. As a result, an economic process which significantly reduces the amount of oil shale which must be handled and treated to yield a given amount of kerogen and which significantly reduces the amount of polluting shale waste would be advantageous.
Various oil shale beneficiating procedures have been proposed. Those separations most proposed are predicated on the differential occurrence of kerogen in the various lumps, pieces and particles of oil shale following the various methods of size reduction and comminution. Since the larger pieces in a reduced shale tend to have a higher kerogen content, simple screening can effect a beneficiation, as described in U.S. Pat. No. 3,133,010. Since kerogen-rich particles possess a lower specific gravity, gravity separation in a dense liquid can also effect a moderate separation, as also mentioned in the reference above. Since kerogen-rich particles differ in wettability from kerogen-poor particles, separation in an aqueous medium by froth flotation is also a significant means of segregating kerogen from the inorganic mineral matter, one method of which is described in U.S. Pat. No. 3,973,734. However, to date none of these proposed oil shale beneficiation procedures has been proven wholly economically effective.
In an acceptable beneficiation procedure, a substantial portion of the inorganic mineral matter will be segregated in a kerogen-poor phase which can be discarded without significant loss of kerogen, and a kerogen-rich phase of substantially reduced weight for kerogen recovery. For example, a beneficiation procedure in which less than 10 percent of the kerogen is discarded and in which the kerogen-rich portion is less than 25 percent of the beneficiation feed would be regarded as a substantial accomplishment.